1. Field of the Invention
The present invention relates to a carrier for an electrophotographic developer used in a copying machine, a printer, etc., for electrophotography, a process for preparing the same and a developer prepared by using said carrier.
2. Prior Art
Various carriers have hitherto been proposed, and examples thereof include those disclosed in U.S. Pat. Nos. 2,618,551, 2,638,416, 2,618,552, 3,526,533, 3,533,835 and 3,591,503.
Further, resin-coated carriers comprising carrier nucleus particles coated with various resins have been proposed for the purpose of prolonging the service life and regulating the amount of charge and resistance of the carrier. Materials such as nonmetals, metals and metal alloys, such as sand, cobalt, iron, copper, nickel, zinc, aluminum, brass, glass and ferrite, and composite metal oxides have been used as the carrier nucleus particle to be coated with the above-described resins.
Although a toner having a mean particle diameter of about 10 to 20 .mu.m has hitherto been used as a developer, there is a tendency that the demand for a high image quality makes it necessary to reduce the particle diameter of the toner. With a reduction in the particle diameter of the toner, it is necessary to increase the specific surface area of the carrier with a view to increasing the capability of imparting a charge to the toner. In the conventional carrier, since the mean particle diameter is about 50 to 150 .mu.m and the surface area is small, the increase in the capability of imparting a charge to the toner is unsatisfactory.
Although carriers in an irregular form having a mean particle diameter of 35 to 50 .mu.m have been known, they are remarkably poor in the fluidity due to their irregular forms, so that the capability of imparting a charge to the toner is unsatisfactory.
In order to improve the fluidity, an attempt has been made on the sphering of the carrier, and atomizing granulation or the like is known as a means for attaining this purpose. However, it has been practically impossible to prepare a spherical powder having a size of 50 .mu.m or less in a high yield: